The bovine parainfluenza virus type 3 (PIV3) vaccine that was developed to prevent disease caused by human PIV3 was safe and immunogenic in seronegative infants and young children 2-36 months of age and has been licensed to a pharmaceutical manufacturer. The live attenuated cold-passaged (cp) human parainfluenza virus type 3 (PIV3) vaccine (cp45) grown in simian FRhL-2 cells (a certified substrate for human vaccines) is safe, poorly transmissible, phenotypically stable, and immunogenic in seronegative infants and children older than 6 months of age. In one to two month old infants, a dose of 5.0 log 10 of cp45 virus was safe and the virus replicated to the same level observed in older seronegative vaccinees, but the serum antibody response was considerably less then that seen in the older vaccinees. Future Phase I through III studies will be performed with cp45 virus grown in simian Vero cells (a licensed substrate for human vaccines) by our CRADA partner. We have identified live attenuated RSV subgroup A candidate vaccines, specifically, the cold-passaged (cp), temperature sensitive (ts), RSV A2 cpts 248/404 and cpts 530/1009 mutants, that are safe, immunogenic, and phenotypically stable in a limited number of seronegative vaccinees. These RSV cpts viruses have been shown to contain non-ts as well as ts mutations that contribute to their attenuation phenotype. This is the first time that a live attenuated RSV vaccine has been found to be safe and phenotypically stable in fully susceptible vaccinees. Expanded Phase I studies are being conducted this year with the cpts 248/404 and cpts 530/1030 candidate vaccines. Studies involving live attenuated subgroup B RSV vaccines were initiated this year, but the two candidate vaccines evaluated thus far in fully susceptible infants and children appear to be overattenuated.